1. Field of the Invention
The present invention relates to an image forming device for use in an image forming apparatus such as a printer, a facsimile, and a copier, and an image forming apparatus including the image forming device.
2. Description of the Related Art
As conventional image forming apparatuses, image forming apparatuses in which a direct recording type image forming method (hereinafter, referred to as direct recording type image forming apparatus) such as so-called toner-jet, direct toning, or toner projection is applied have been known (such as Japanese Patent Publication No. 2910019). In the image forming apparatuses of this type, unlike an electrophotographic process in which a latent image formed on a photosensitive body is developed by an image forming agent such as toner and the developed image is transferred from the photosensitive body onto a recording paper or a transfer medium, an image is directly formed on a recording paper, transfer medium, and the like, by depositing an image forming agent (toner) caused to be sprayed on the recording paper, the transfer medium, and the like.
FIG. 19 is a schematic of a key portion of a conventional direct recording type image forming apparatus. In FIG. 19, an agent carrier 501 is arranged so that the axis line is extended in the horizontal direction in FIG. 19, and while rotated and driven by a drive unit, which is not shown, the agent carrier 501 carries toner T that has been charged as an image forming agent on the surface. A flexible print circuit (FPC) 503 that is a hole defining member defining a plurality of holes 502 therein is arranged below the agent carrier 501. The FPC 503 includes a plurality of spray electrodes 504 in a ring shape formed on the surface opposite to the agent carrier 501, to surround each of the holes 502.
Below the FPC 503, a counter electrode 506 placed opposite to the agent carrier 501 with the FPC 503 interposed therebetween and a recording paper 507 placed on the counter electrode 506 and conveyed by a conveying unit are arranged. In FIG. 19, for descriptive purposes, only one each of the holes 502 and the spray electrodes 504 are shown. However, in practice, a plurality of combinations of the hole 502 and the spray electrode 504 is formed on the FPC 503. More specifically, for example, 4960 combinations are formed on the FPC 503 for 600 dots per inch (dpi).
The agent carrier 501 carries the toner T charged to a minus polarity on the surface, for example, while being grounded. When spraying voltage of plus polarity is applied to the spray electrodes 504, an electric field having a predetermined strength is applied to the toner T on the agent carrier 501 placed opposite to the spray electrodes 504, and the toner T placed near the toner T on the agent carrier 501 placed opposite to the spray electrodes 504. Due to the action of the electric field, the electrostatic force applied to the toner T exceeds the adhesive force between the toner T and the agent carrier 501. Accordingly, an aggregation of the toner T selectively caused to be sprayed from the agent carrier 501 in a dot shape, enters the holes 502. The toner T is caused to continuously be sprayed while being attracted to an electric field formed between the spray electrodes 504 and the counter electrode 506 charged to a potential higher than that of the spray electrodes 504. The toner T then passes through the holes 502, deposits on the surface of the recording paper 507, thereby forming a dot image.
The ON-OFF of the spraying voltage applied to each of the spray electrodes 504 is individually controlled by a dedicated integrated circuit (IC). In general, more chip area is required for IC, with an increase in the withstand voltage, thereby requiring a certain extent of installation space. Accordingly, the IC is fixed on an electrical substrate, which is not shown, integrally connected with the FPC 503, and the electric substrate is installed at a position a short distance away from the FPC 503.
To obtain high quality images in which the density and resolution of dot images is enhanced in the direct recording method, a toner supply gap that is an interval between the agent carrier 501 and the FPC 503 must be set and maintained with high accuracy.
In the image forming apparatus disclosed in Japanese Patent Publication No. 2910019, a positioning member that comes into contact with a hole defining member defining the holes therein and positions the hole defining member relative to the agent carrier, is fixed to both ends in the axial direction of the agent carrier. Because the positioning member positions the hole defining member relative to the agent carrier, a predetermined toner gap that is an interval between the agent carrier and the hole defining member can be set.
However, in the image forming apparatus disclosed in Japanese Patent Publication No. 2910019, the hole defining member only comes into contact with the positioning member at small portions of both ends in the axial direction of the agent carrier. The hole defining member is positioned under a so-called both ends supported state. Accordingly, a region near the center of the hole defining member in the axial direction of the agent carrier tends to bend and distort, by the weight of the hole defining member. Particularly, if the hole defining member is made of a flexible material such as a flexible print circuit, the bending and distortion near the center of the hole defining member in the axial direction of the agent carrier become significant. Due to such bending and distortion are produced on the hole defining member, problems such as the toner supply gap fluctuates in the axial direction of the agent carrier, and a predetermined toner supply gap cannot be maintained in the axial direction of the agent carrier occur.
The applicants of the present invention have proposed an image forming device that can solve the problems in Japanese Patent Application Laid-open No. 2008-273508 (hereinafter, referred to as prior application).
In other words, the image forming device of the prior application includes a casing that stores therein an agent, an agent carrier rotatably supported by the casing and facing outside through an opening formed on the casing, a sheet-like hole defining member that defines a plurality of holes therein, and placed opposite to the agent carrier with a predetermined interval therebetween, and a plurality of spray electrodes that is provided on the hole defining member in a manner corresponding to the holes, and forms an electric field for selectively causing the agent to be sprayed from the agent carrier towards the holes. The image forming device also includes a positioning member that is provided on the casing so as to cover the agent carrier, at least holds the hole defining member in the axial direction of the agent carrier by the side walls of the positioning member having an opening at a location opposite to a plurality of toner holes in the hole defining member, and positions the holes relative to the agent carrier so that the relative positions of the agent carrier and the holes are in a predetermined positional relationship.
In the image forming device of the prior application, the positioning member holds the hole defining member by the side walls in the axial direction of the agent carrier. Accordingly, compared to when the hole defining member is only held by the positioning member by both ends in the axial direction of the agent carrier, it is possible to prevent the hole defining member from bending, for example, causing the fluctuations in interval in the axial direction of the agent carrier. Consequently, it is possible to prevent the interval between the agent carrier and the hole defining member from fluctuating in the axial direction of the agent carrier, thereby maintaining a predetermined interval.
If the printing gap that is an interval between the holes formed on the hole defining member and the recording member fluctuates, the spraying time of the toner caused to be sprayed from the holes towards the recording member will not be uniform. Accordingly, the recording characteristics (image qualities) deteriorate significantly. The fluctuation of the printing gap is caused because, when the recording member passes through the location opposite to the hole defining member, the recording member moves in the thickness direction of the recording member while being displaced, due to vibration and the like.
For example, WO01/032432 discloses an image forming apparatus in which an image forming device is so arranged that, to maintain a predetermined printing gap, an intermediate transfer belt (i.e., a recording member) is brought into contact with a counter electrode including an abutting unit with which the intermediate transfer belt is wound and brought into contact, and part of the intermediate transfer belt that is wound around and brought into contact with the counter electrode and a plurality of holes of the hole defining member are opposed to each other.
Because the intermediate transfer belt is wound around and brought into contact with the counter electrode, the intermediate transfer belt and the counter electrode are brought into contact with each other without fail. Accordingly, it is possible to prevent the intermediate transfer belt from displacing in the thickness direction thereof, thereby maintaining a predetermined printing gap that is an interval between the holes of the hole defining member and the intermediate transfer belt.
FIG. 20 is a schematic of a color image forming apparatus on which four image forming devices 120 having the same configuration that form toner images of different colors on an intermediate transfer belt 200 by causing toner to be sprayed from a plurality of holes formed on a hole defining member 104, are mounted.
In this image forming apparatus, as described above, the intermediate transfer belt 200 is wound around and brought into contact with counter electrodes 240 corresponding to the image forming devices 120. In general, the locations where the intermediate transfer belt 200 is wound around the counter electrodes 240 tend to deviate towards the upstream or downstream of the counter electrodes 240 in the moving direction of the intermediate transfer belt, and a degree of deviation often differs. Accordingly, if the four image forming devices 120 having the same configuration are so arranged in the image forming apparatus that part of the intermediate transfer belt 200 that is wound around and brought into contact with the counter electrode 240 is opposed to a plurality of holes of the hole defining members 104, the image forming devices 120 are arranged in the image forming apparatus having different orientations for each of the counter electrodes 240 as shown in FIG. 20. Accordingly, if the image forming devices 120 are arranged in different orientations, as evident in FIG. 20, the image forming devices 120 are spread in a fan shape about the intermediate transfer belt 200, thereby forming unnecessary space between the image forming devices. Accordingly, the size of the image forming apparatus main body is increased.
To arrange the image forming devices 120 in the same orientation in the image forming apparatus, the configuration of the image forming device 120 may be varied, by changing the positions of the holes formed on the hole defining member 104 by each image forming device 120, and the like. However, if each of the image forming devices is formed differently, members dedicated for each of the image forming devices are required, thereby increasing cost.